The invention relates to a process for the reconstruction of three-dimensional images of an area of interest of an object and involves a combination of measurements performed on the complete object with measurements performed on an area of interest of the object, as well as to an appropriate installation.
Processes are already known in which there is only an interest in the reconstruction of bidimensional images on an area of interest of a more voluminous object, such as an organ in a human body. In the simplest procedure a series of unidimensional measurements are carried out on sections at different orientations around the object and then the results of the measurements are combined in an appropriate manner, as if it were necessary to reconstruct the image of the entire object. However, the unidimensional (linear) array of detectors responsible for carrying out the measurements is then disposed so as to substantially only receive radiation emitted by a source, which has traversed the area of interest of the object in accordance with the considered section (or in an equivalent manner radiation emitted by said area of interest). The reconstruction of bidimensional images is possible when a sufficient number of views have been taken by rotating the array of detectors around the object and then the three-dimensional images are obtained by superimposing bidimensional images taken on adjacent sections. However, for each measurement, there is a contribution of portions of the object located outside the area of interest, either in front of or behind the object in the field of vision of the detectors, and these portions are different for each orientation of the detectors, because there are no measurements on those portions of the object located on either side of the area of interest. Such measurements are called truncated measurements of the object and are due to inevitable reconstruction errors, unlike in the case of processes where the viewing field of the detectors takes in the entire object. However, taking in the entire object this would increase the complexity of the detector, its overall dimensions and its cost, or would lead to a less good resolution of the image of the area of interest if the same detector was retained.
Various methods have already been proposed in the case of a planar, fan-shaped radiation and a reconstitution of the object by superimposed parts in order to correct at least partly the truncation errors. In one of these methods, described in British patent 2 088 670, two series of measurements are performed: measurements across the area of interest of the object and measurements performed through the entire object, at a much lower radiation intensity so as to not cause excessive irradiation. The second series of measurements supplies informations at low frequencies or low resolution on the object, which make it possible to complete the images of the area of interest obtained as a result of the first series of truncated measurements. Other processes consist of introducing into the measurement combination calculations or estimates of results for portions of the object excluded from the area of interest, or simply the position of the external contour of the object, followed by the "extrapolation" of the radiation emission or attenuation sums collected during the measurements over the entire surface enclosed by the contour (article by Ogawa, Nakajima and Yuta entitled "A reconstruction algorithm from truncated projections", published in IEEE Transactions on Medical Imaging, vol. MI-3, No. 1, March 1984, pp 34 to 40).